Screw adjustment mechanism with pre-set backlash
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to screw adjustment mechanisms for ink fountain blades and the like, and more particularly to a screw adjustment mechanism wherein the amount of looseness (thread backlash) between two threadedly mated parts can be easily adjusted to a desired minimum. 
     2. Description of the Prior Art 
     Threaded shafts (screws) are much used to change the positional relationship between two or more parts. Normally one of such parts is longitudinally movable but held against rotation about a mating screw, and hence is displaced axially of the screw and moved with respect to the other part on turning of the screw. Unfortunately, the amount of looseness of fit (thread backlash) that occurs between mating threaded parts is unpredictable, with the result that the adjusted relationship between the two parts can not be made as precise as is desirable without resort to expensive components. 
     But even expensive components are not a complete answer to the problem. Wear and tear resulting from use of the screw adjustment mechanism eventually results in a looseness of fit which makes precise adjustment of the part impossible. The problem becomes acute when different metals are in contact, and particularly so when one of the metals is a soft one like aluminum of which ink fountain blades are frequently made. 
     Existing technology includes many ways of adjusting for thread backlash where the threaded components act as an &#34;in and out&#34; adjustment of a mechanism. All methods employed to date require an assembly procedure which either requires extreme accuracy of the mating parts, or a calibrating adjustment to be made after final assembly of the constituent parts. All existing methods of accomplishing the desired results are either too expensive or difficult to adjust to allow their use in a predictable manner when many such assemblies are required. With the increased use of computer control of adjusting mechanisms, the need for reliable, inexpensive means of achieving improved fit in threaded mechanisms is rapidly exceeding the practicality of the means of achieving it. 
     Particular prior art includes U.S. Pat. Nos. 4,000,695; 4,655,133; 1,275,348; 2,583,640; 4,534,290; 4,581,994; 3,559,573; 3,730,090; and 4,058,058. U.S. Pat. No. 4,000,695 (Perretta) shows an ink fountain blade assembly wherein a screw 16 is used to adjust the matingly-threaded ink fountain blade 13 with respect to the assembly frame. U.S. Pat. No. 4,655,133 (O&#39;Dea et al) shows the use of a spring 34 about a screw 36 to compensate yieldably for backlash. Showing similar arrangements are U.S. Pat. No. 1,275,348 (Wood); U.S. Pat. No. 2,583,640 (Faeber); U.S. Pat. No. 4,534,290 (Schroder et al); and U.S. Pat. No. 4,581,994 (Wildman). U.S. Pat. No. 3,559,573 (Hantscho) shows a differentially threaded screw 26 matingly engaging two threaded parts 20 and 22, but no backlash adjustment mechanism. U.S. Pat. No. 3,730,090 (Lamberg) also shows a differential thread assembly, but no backlash adjustment mechanism. U.S. Pat. No. 4,058,058 (Hantscho) shows use of a set screw 56 to hold a screw 41 in place with respect to its mating member, and use of a set screw 52 extending into a recess 54 to limit longitudinal movement of a blade 37. 
     SUMMARY OF THE INVENTION 
     A main object of the invention is to provide a screw adjusting mechanism for ink fountain blade assemblies and the like, wherein the backlash between parts threadedly mated, can be adjustably limited. 
     Another object of the invention is to provide such a screw adjustment mechanism that allows easy adjustment of the amount of desired backlash prior to final assembly of the mating threaded parts. 
     Another object of the invention is to provide such a screw adjusting mechanism wherein the constituent parts can be inexpensive to manufacture. 
     Still another object of the invention is to provide such a screw adjusting mechanism that permits assembly without complicated calibration. 
     Yet another object of the invention is to provide such a screw adjustment mechanism that allows subsequent readjustment to compensate for wear between the threaded parts. 
     These objects of the invention are achieved through the use of a spring between two bushings threadedly mated to a screw and locking the bushings to one of two parts to be moved relative to each other on turning of the screw. The spring causes the bushings to be oppositely situated, threadwise, in their relationships to the screw, before they are locked firmly in place with respect to the part to be adjusted and to each other. Lost motion possibilities between the screw and the part to be adjusted are thus reduced if not eliminated. 
     A feature of the invention is that the tightness of the fit can be increased or decreased by the proper selection of the size or constant of a spring, or by the amount that the spring is tensioned. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects, advantages and features of the invention will be apparent from a consideration of the following description of an illustrative embodiment of the invention when taken together with the accompanying drawings wherein: 
     FIG. 1 is a schematic sectional view of a screw adjustment mechanism constructed according to the invention. 
     FIG. 2 is a pre-assembly, partly exploded view of the mechanism of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a screw adjusting mechanism constructed according to the invention is shown as including a threaded stainless steel shaft 1 fitted with two threaded bronze bushings 3 and 5 respectively. The bushings are separated on the shaft 1 which mounts there between a steel coil spring 7 that is compressed to approximately one-half its free length. The bushings are snugly located in a hole 19 in a longitudinally moveable part 9 which may be an aluminum alloy ink fountain blade. Set screws 11 and 13 threaded in the part 9 engage the bushings 3 and 5 respectively to hold the bushings against movement with respect to part 9. 
     The screw adjusting mechanism of FIG. 1 would be adjusted for backlash prior to final assembly. Assembly may be begun by first screwing bushing 5 on one end of the stainless steel shaft 1, then placing the coil spring 7 on the shaft, and then threading on the shaft the bushing 3. The bushing 3 will be screwed on shaft 1 to compress a desired amount the coil spring 7 between it and the other bushing 5. Compression of the coil spring 7 causes it to exert axial forces on the bushings which results in the right side (FIG. 2) of the thread of bushings 5 resting against the left side of the thread of the stainless steel shaft 1 at point 17. It also results in the left side of the thread of bushing 3 resting against the right side of the thread of the stainless steel shaft 1 at point 15. It will be evident that any looseness between either of the bushings and the shaft is now caused by the coil spring 7 to be taken in yielding engagement. It will also be evident that this yielding engagement is opposite for the two bushings. It will be evident too that axial forces urged by the compression spring and hence the forces acting between the bushing and the shaft threads can be varied by choosing springs of different sizes and constants, and by varying the location of the bushings with respect to each other on the shaft. 
     Assembly of the screw adjustment mechanism is completed by inserting the end of the steel shaft 1 with its bushings and coil spring fitted in place into the opening 19 formed in the longitudinally movable part 9. The shaft is inserted to a depth which permits threaded bushing 3 to fall adjacent set screw 11 and threaded bushing 5 to fall adjacent set screw 13. With the coil spring 7 continuing to keep the bushings 3 and 5 in intimate contact with the shaft 1 at points 15 and 17, the set screws 3 and 5 are tightened to secure bushings 3 and 5 fixedly in place with respect to the longitudinally movable part 9. In fixing the bushings with respect to the part, the looseness of bushings with respect to the shaft 1 is also fixed. Basically, the entity of the part 9 acting through the two bushings 3 and 5 is now in firm engagement with each side of the thread on shaft 1, essentially eliminating any looseness (threaded backlash). 
     After assembly, the shaft 1 remains free to rotate smoothly with a minimum of backlash relative to the mating part 9. Should it be or become desirable to change the tightness of the fit, adjustment may be made by loosening either of the set screws 11 and 13 and turning the shaft to advance it and the (now) loose bushing towards or away from the still fixed bushing to increase or decrease spring 7 compression and hence the axial forces acting on the threads. The loosened set screw would thereafter be tightened. Of course, both set screws 11 and 13 may be loosened to allow removal of the shaft 1, bushings 3 and 5, and spring 7, to enable external adjustment of the thread backlash. 
     It should be apparent that applicant has provided for the precise adjustment of backlash between two mating threaded parts used in a control mechanism without requiring constituent parts that are costly of manufacture. Furthermore that assembly can be made without complicated calibration. Moreover, applicant&#39;s invention allows subsequent readjustment of the tightness of fit in a simple uncomplicated manner. It should also be observed that the relatively soft aluminum alloy part is effectively isolated from thread contact to effectively neutralize it as a source of looseness of fit. 
     It will be appreciated that the foregoing is to be considered as illustrative only of the principles of the invention, and that while certain novel features of the invention have been shown and described, various omissions, substitutions and changes in the form and detail of the mechanism illustrated and in their use and operation can be made by those skilled in the art without departing from the spirit of the invention.